The present invention relates to capacity modulation of compressors. More particularly, the present invention relates to a diagnostic system for a capacity modulated compressor which is capable of determining if the capacity modulation system is functioning properly.
Capacity modulation is often a desirable feature to incorporate in air conditioning and refrigeration compressors in order to better accommodate the wide range of loading to which the systems may be subjected. Many different approaches have been utilized for providing this capacity modulation feature ranging from controlling of the suction inlet to bypassing discharge gas back to the suction inlet. With scroll-type compressors, capacity modulation has often been accomplished via a delayed suction approach which comprises providing ports at various positions which, when opened, allow the compression chambers formed between the intermeshing scroll wraps to communicate with the suction gas supply thereby delaying the point at which compression of the suction gas begins. This method of capacity modulation actually reduces the compression ratio of the compressor. While such systems are effective at reducing the capacity of the compressor, they are only able to provide a predetermined amount of compressor unloading, the amount of unloading being dependent upon the positioning of the unloading ports along the wraps. While it is possible to provide multiple step unloading by incorporating a plurality of such ports at different locations, this approach becomes costly and requires additional space to accommodate the separate controls for opening and closing each set of ports.
Other capacity modulation systems overcome these deficiencies in that they enable virtually a continuous range of unloading from 100 percent or full capacity down to virtually zero capacity utilizing only a single set of controls. Further, these systems enable the operating efficiency of the compressor and/or refrigeration system to be maximized for any degree of compressor unloading desired.
In these capacity modulation systems, compressor unloading is accomplished by cyclically effecting axial or radial separation of the two scroll members for predetermined periods of time during the operating cycle of the compressor. More specifically, an arrangement is provided wherein one scroll member is moved axially or radially toward and away from the other scroll member in a pulsed fashion to cyclically provide a leakage path across the tips or flanks of the wraps from higher pressure compression pockets defined by the intermeshing scroll wraps to lower pressure pockets and ultimately back to suction. By controlling the relative time between sealing and unsealing of the scroll wrap tips or flanks, virtually any degree of compressor unloading can be achieved with a single control system. Further, by sensing various conditions within the refrigeration system, the duration of compressor loading and unloading for each cycle can be selected for a given capacity such that overall system efficiency is maximized. For example, if it is desired to operate the compressor at 50 percent capacity, this can be accomplished by operating the compressor alternately in a loaded condition for five seconds and unloaded for five seconds or loaded for seven seconds and unloaded for seven seconds, one or the other of which may provide greater efficiency for the specific operating conditions being encountered.
The various capacity modulation systems all have the capability of reducing the capacity of the compressor and all work well within the design limits of the particular system. While the capacity modulation systems function in an acceptable manner, there is a need to be able to determine if and when these systems have stopped functioning properly.
The present invention provides a simple low-cost system which is capable of detecting the failure of a capacity modulation system. In a capacity modulation system which opens and closes a fluid passage between two areas of the compressor utilizing a valve, the proper functioning of the system can be accomplished by monitoring the fluid temperature downstream of the valve. If the valve fails, either open or closed, the temperature in the downstream passage will be steady as opposed to fluctuating with the opening and closing of the valve during reduced capacity modulation. Knowing this downstream temperature also allows for the detecting of whether the valve failed in an open or closed position since this temperature would have two different valves for these two failure modes. Another approach is to sense the temperature differential between upstream and downstream of the valve. This temperature value coupled with the temperature error in the room provide effective conformation of these failing modes.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.